1. Field of the Invention
This invention relates to underwater offshore storage of liquids such as oil and, in particular, it concerns novel undersea oil storage arrangements characterized by economical, lightweight construction and nonpolluting operation.
2. Description of the Prior Art
Underwater offshore oil storage facilities have previously been proposed for various applications. Examples of such proposals are given in U.S. Pat. Nos. 3,322,087; 3,408,971; 3,695,047 and 3,943,724. It has been found that when such storage facilities are to be provided at large water depths and are to have large storage capacities, very substantial forces are produced on their wall surface by the action of waves and tides and by the pressure differentials which occur as oil is transferred into and out from their interior. In some instances, for example, as shown in U.S. Pat. Nos. 3,322,087 and 3,408,971, the oil contained within the device floats on a layer of water which is in open communication with the sea. While this serves to help minimize pressure differentials across the walls of the structure, it raises a danger of pollution because oil mixes with the layer of water on which it floats and may thus easily escape to the sea. U.S. Pat. No. 3,943,724 proposes to provide a flexible membrane between the oil and the water in an undersea storage tank but such membranes are expensive, unreliable and not suited to very large installations. As a result it has been necessary in the past to build large undersea oil storage facilities of very heavy reinforced concrete to ensure that the oil containing compartments were isolated from the sea and at the same time to withstand the large forces produced by the sea on the walls of the oil compartments. These concrete structures were expensive to manufacture; and, because of their great weight, their installation was also very difficult and expensive.
U.S. Pat. No. 3,893,918 proposes a separator conduit or "skim pile" for containing oil and separating it from water at an offshore location; but it provides no indication as to how the oil storage problem described above can be solved.